The present invention relates to a coil apparatus and, in particular, to a winding arrangement of a coil apparatus comprising two overlapped coil bobbins. The present invention can be utilized as a power converting transformer in a switching power supply circuit.
FIG. 1 is a cross sectional view of a conventional coil apparatus having a single coil bobbin. This coil bobbin 1 has a hollow cylindrical or rectangular member and flanges located on both the extreme ends of the hollow member. A primary winding P is fitted on the hollow member. An insulation film 2 is provided around the primary winding P. Furthermore, a secondary winding S is fitted on the film 2. In this arrangement, the primary winding P is electrically coupled with a switching circuit (not shown) and the secondary winding S is electrically coupled with a load (not shown). Of course, a core (not shown) made of magnetic material is inserted into the hollow member.
FIG. 2 is a cross sectional view of another conventional coil apparatus using two overlapped coil bobbins. One coil bobbin 3, referred to as an inner coil bobbin, has a hollow cylindrical or rectangular member and two flanges on both the extreme ends thereof. The other coil bobbin 4, referred to as an outer coil bobbin, has also a hollow cylindrical or rectangular member and two flanges on both the extreme ends thereof. As apparent from FIG. 2, the diameter of the inner bobbin 3 is smaller than that of the outer bobbin 4. Then, the inner bobbin 3 is inserted into the hollow member of the outer bobbin 4. A primary winding P is fitted on the hollow member of the inner bobbin 3, a secondary winding S being fitted on the hollow member of the outer bobbin 4. In the same way as the apparatus of FIG. 1, a core (not shown) is inserted into the cylindrical member of the inner bobbin 3.
When the above-mentioned coil apparatuses are utilized in a power supply circuit, the following conditions must be satisfied.
(a) The Curie temperature of the magnetic core is sufficiently high and its maximum magnetic flux density is large.
(b) The iron loss is small in a high frequency band.
(c) The temperature characteristics of the magnetic core are satisfactory.
(d) The copper loss of the windings is small.
(e) The magnetic coupling between the windings is satisfactory.
(f) The insulation breakdown strength between windings must exceed that requested by the safety standard issued in each country.
The above conditions (a) through (c) depend on the property of the magnetic core and the condition (d) depends on both the diameter and the length of windings. Furthermore, the condition (e), depending on the winding manner or winding structure of the coil apparatus, is closely related not only to the electric performance of the coil apparatus but to the performance of the power supply circuit. For instance, with the coil apparatus used as the power converting transformer in the switching power supply circuit, if the magnetic coupling between windings is low, the leakage flux will increase and thus the leakage inductance will become large. Therefore, an energy which is stored in the leakage inductance while a switching element coupled with the primary winding is in the ON state is discharged just when the switching element changes from ON to OFF. The discharge of the stored energy induces a high voltage across the primary winding, thus causing the switching element to be damaged. Furthermore, this discharge due to the low magnetic coupling, brings about the vibration of switching waveforms which affects the switching operation, the increase of the loss and the decrease of its efficiency.
In order to improve the magnetic coupling between windings, many types of the winding manners of the coil apparatus using a single bobbin as shown in FIG. 1 have been proposed. As examples of those winding manners, there are so-called a voltage-dividing manner, a current-dividing manner, a sandwich manner and so on. FIG. 3 is a circuit diagram of a coil apparatus according to the voltage-dividing manner. As shown in FIG. 3, this apparatus has two primary windings P.sub.1 and P.sub.2 and a secondary winding S. The primary winding P.sub.1 is divided into two windings P.sub.11, P.sub.12. One end of the winding P.sub.1 is connected to a terminal 6, and the other to a terminal 7. One end of the winding P.sub.12 is connected to a terminal 8, the other to the terminal 7. In this arrangement, the voltage V.sub.1 applied across the terminals 6 and 8 is divided by two windings. The primary winding P.sub.1 composed of two windings P.sub.11, P.sub.12 in a series, the other primary winding P.sub.2 (which is terminated at terminals 9, 10) and the secondary winding S (which is terminated at terminals 11, 12) are disposed for the single bobbin, as shown in FIG. 4. As apparent from this figure, the secondary winding S and the primary winding P.sub.2 are arranged so as to be interposed between two divided windings P.sub.11 and P.sub.12. An insulation film 14 is provided between the windings S and P.sub.2 and an insulation film 15 is provided between the windings S and P.sub.11 to insulate between adjacent windings.
The winding arrangement of FIG. 4 can provide the improvement of the magnetic coupling between the primary side and the secondary side. However, it is difficult to ensure a sufficient insulation voltage therebetween for satisfying the safety standards by means of the insulation films 14, 15.
On the other hand, the prior coil apparatus having two overlapped bobbins has the winding arrangement such that the primary windings P.sub.1, P.sub.2 are wound on the inner bobbin 3 in this order and the secondary windings on the outer bobbin 4. This arrangement can provide a sufficient insualtion breakdown strength between primary side and the secondary side for satisfying the safety standards. However, as compared with the arrangement of the coil apparatus of FIG. 4, the magnetic coupling between primary side and the secondary side is small and thus the leakage inductance is large because the length between the primary side and the secondary side is rather long. Furthermore, it is impossible for the coil apparatus having two overlapped bobbins to have the winding arrangement of the coil apparatus having a single bobbin of FIG. 4 in order to improve the magnetic coupling. Therefore, an winding arrangement for increasing the magnetic coupling between the primary coil and the secondary coil has not been proposed yet.